Continuing and extending present studies on isolated gastric parietal cells from rabbits, we shall investigate conditions for optimal observation of H ion transport, and quantitate the effects of the secretagogues histamine, carbachol, pentagastrin and caerulein and the inhibitors metiamide, secretin and prostaglandin D1. In light of evidence from other laboratories that respiration of isolated parietal cells is stimulated by agents that promote gastric acid secretion and blocked by agents that inhibit secretion, we shall reexamine our tentative binding that these cells do not so respond, and that only succinate supports even a minimal respiratory rate. Finally, we shall investigate the importance of extracellular calcium in regulation of acid transport and respiration of parietal cells.